Regulated DC power supplies are typically employed in analog and digital electronic systems. Two major categories of regulated DC power supplies are linear power supplies and switching power supplies. For reasons that will become more apparent, switching power supplies are generally the power supply of choice. In switching power supplies, transformation of a DC voltage from one level to another is often accomplished with a DC/DC converter, such as a step-down (buck) or step-up (boost) converter. Solid state devices, such as transistors, are operated as switches (either completely ON or completely OFF) within the switching converters. Since the power devices do not operate in the active region (as dictated in linear power supplies), the switching power supplies can achieve lower power dissipation in comparison to linear power supplies. Furthermore, the higher switching speeds and voltage and current ratings of the presently available power devices has further expanded the popularity of switching power supplies.
For applications that require three phase off-line rectification with low input current total harmonic distortion (THD), the simplest switching power converter topology is a single switch discontinuous current mode (DCM) boost converter. By operating the input inductors of the boost converter in DCM, at the beginning of each switching cycle, i.e., when the boost switch is ON, currents through the input inductors begin to ramp up from an initial value of zero at a rate proportional to the corresponding phase-to-neutral voltage of the input inductors. Consequently, the average input inductor currents, which are also the phase currents, are naturally proportional to the corresponding phase voltages when the boost switch is ON.
When the boost switch is OFF, an output voltage which is higher than the peak of the input phase-to-phase voltage (due to the operation of the boost converter) drives the currents through the input inductors to zero prior to the initiation of the next switching cycle. The rate of decrease of the current through each input inductor, however, is not proportional to the corresponding input phase-to-neutral voltage due to the participation of the output voltage. Thus, when the boost switch is OFF, the average input inductor currents, in this period, are not proportional to the corresponding phase-to-neutral voltages of the input inductors resulting in distortions in the input phase currents. It should be noted that for higher output voltages, the currents through the input inductors decrease at a faster rate thereby reducing distortions in the input currents.
Studies have shown that to achieve an input current THD of less than 10%, for instance, the output voltage of the boost converter should be about 1.7 times greater than the input phase-to-phase peak voltage (M&gt;1.7; where M=Vout/Vin(peak)) of the boost converter. The high output voltage requirement to attain a low input current THD normally results in an output voltage that is higher than what is generally desired. For example, for an input voltage of 208 Vrms and taking into account the presence of input voltage fluctuations, the output voltage would have to be about 650 volts (significantly higher than a conventional 400 V output) to realize an input current THD of less than 10%. For an input voltage of 440 Vrms, the output voltage would then have to be as high as 1300 volts (far above a conventional 800 volt output) to achieve an input current THD of less than 10%.
Accordingly, what is needed in the art is an improved power converter that overcomes the above described limitations. More specifically, what is needed in the art is a power converter that can achieve a low input current THD and, at the same time, maintain the output voltage of the converter within a preferable range.